1. Field of the Invention
This invention relates generally to stored program controlled electronic telephone switching systems and, more particularly, to the maintenance of data and control signal integrity between a central host office in such a system and one or more remote offices.
2. Description of the Prior Art
A digital time division telephone electronic switching system (ESS) utilizing stored program control broadly comprises a central processor (CP), a message switch (MSGS), a time-multiplexed switch (TMS) and one or more interface modules (IM). The CP provides central control of all switching functions as well as memory, bulk storage and data link interfaces. The MSGS provides duplicated control and message data transfer between the CP at a Host Office and the IMs. The TMS is a time-shared space division switch providing routing paths for control and data messages in pulse-code modulation (PCM) format among the IMs. The IMs terminate telephone subscriber lines and trunks from other telephone central offices. Analog signals are converted into the PCM format within the IMs. A time-slot interchange (TSI) unit within each IM performs time-division switching while the TMS performs space division switching. The signaling and data paths between IMs and the TMS are affected through four Network, Control and Timing (NCT) links, (two of which are active) and may be implemented using fiber optic technology. An NCT link provides 256 time slots (TS), each of which contains sixteen bits sampled at a baud rate of 8000. A frame of 256 TS with 16 bits per TS has a length of 4,096 bits and the overall high-speed data rate therefore becomes 32.768 million bits per second (Mb/s).
A need has arisen to effect a connection between a host central office and a remotely located IM over a plurality of PCM transmission facilities known by the Bell System designation as T1 lines. By way of example, up to ten T1 lines, each providing twenty-four eight-bit PCM channels, are mapped into each of the active NCT links at the maximum capacity of ten T1 lines per link. 230 of the 256 available time lots in each NCT link are available for message traffic and control data. Inasmuch as data are being continually transmitted in both directions between the host and the remote offices, the integrity of the several components in the transmission facilities between the host and remote IMs can be tested continuously and the location and type of error arising assessed. A fixed alternation of parity is assigned to successive transmission channels in one direction of transmission and the same parity is repeated on successive transmission channels in the opposite direction of transmission at the interface between T1 lines and the NCT links. This pattern of parity is repeated every two frames. The pattern sent in the even-numbered frame is the inverse of that in the odd-numbered frame.
It is an object of this invention to provide an automatic fault detection parity scheme for the multiplexed interface between PCM lines and serial time-division links in a two-way data transmission system.